feature request: helper methods palette expressions as python lists

[andy-esch]

Right now the palette keyword takes a string expression, but it would be handy to have it take lists as well:

Map(
    color_category_layer(
        'county_measures',
        'male_or_female', 
        'Predominant Gender', 
        palette=['#6486FD', '#FD8F64', 'yellow']   # in addition to '[#6486FD,#FD8F64,yellow]'
    )
)

[Jesus89]

OK. This is because we pass directly the param to VL, and ['#6486FD', '#FD8F64', 'yellow'] cannot be parsed as a palette because they are strings.

We can add a utility method in the color helpers to serialize the palette to [#6486FD, #FD8F64, yellow].